Characterization of a thermally tolerant Orbicella faveolata reef in Abaco, The Bahamas

Increased ocean temperatures from anthropogenic climate change induce coral bleaching, the breakdown of symbioses between corals and photosynthetic dinoflagellates. However, some corals thrive in marginal, warm environments that exceed typical bleaching thresholds. Their survival may be mediated by specific genes within the coral host, association with heat-tolerant algal symbionts, and/or distinct bacterial communities. At Mermaid Reef in Great Abaco, The Bahamas, Orbicella faveolata colonies did not bleach during a warming event that reached 33.0 °C, while at Sandy Cay Reef (~ 18 km south), which reached only 32.0 °C, O. faveolata bleached extensively. To investigate abiotic and biotic factors contributing to Mermaid Reef’s higher thermal tolerance, we compared temperature, depth, and coral composition at each site and used microsatellite genotyping, quantitative PCR, and 16S rRNA metabarcoding to examine host genotype diversity, Symbiodiniaceae composition, and bacterial communities in O. faveolata. All O. faveolata colonies at the tolerant Mermaid Reef were clonemates and hosted exclusively Durusdinium symbionts, while colonies at Sandy Cay Reef comprised diverse genotypes and hosted varying proportions of four Symbiodiniaceae genera, which were primarily structured by depth. Mermaid Reef colonies also tended to have higher bacterial family richness than Sandy Cay Reef. These findings suggest that shallow, warm environments like Mermaid Reef may select for few, putatively heat-tolerant genotypes of corals and symbionts, and that while warming may greatly reduce genetic diversity, certain individuals may thrive. Such individuals existing today can provide valuable biological insights and resources for intervention conservation aimed at boosting reef resilience under climate change.